During heated hearings in Congress last week, CEO Mark Zuckerberg had already explained that Facebook collects data beyond what users share on their profiles.
"When you visit a site or app that uses our services, we receive information even if you're logged out or don't have a Facebook account," product management director David Baser said in a post on the social network's blog.
"This is because other apps and sites don't know who is using Facebook," he added, noting Facebook was also following up with Congress on a few dozen questions Zuckerberg was unable to answer at the time of the hearings.
Baser said "many" websites and apps use Facebook services to target content and ads, including via the social network's Like and Share buttons, when people use their Facebook account to log into another website or app and Facebook ads and measurement tools.
"Most websites and apps send the same information to multiple companies each time you visit them," the post said.
"I want to be clear: We don't sell people's data. Period."
Zuckerberg says Facebook "failed" to protect people's information following the use by Cambridge Analytica of data scraped from 87 million Facebook users to target political ads ahead of the 2016 US presidential election.
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